I am currently analysing metformin using phenformin as an internal standard using a hilic column. When I run a blank ACN solvent the metformin and phenformin peaks do not appear but when I run a solvent blank of the buffer mobile phases (10 mM ammonium formate:0.1% formic acid in ACN) or (10 mM ammonium formate:0.1% formic acid in water) then the phenformin and metformin peaks return with high intensity. I have checked the buffer solution reservoirs and they are not contaminated. Could the problem lie with the inlet valve or some contamination along the line or MS detector?

Some additional info:

Metformin's retention time is 1.04 mins. MS transitions are 130--->71 and 130---->60

Phenformin's retention time is 0.59 mins. MS transitions are 206.077--->60.200 and 206.077--->105.200.